The field of this invention is selective absorption of CO2 gas.
In many applications where mixtures of two or more gaseous components are present, it is often desirable to selectively remove one or more of the component gases from the gaseous stream. Of increasing interest in a variety of industrial applications, including power generation, chemical synthesis, natural gas upgrading, and conversion of methane hydrates to hydrogen and CO2, is the selective removal of CO2 from multicomponent gaseous streams.
An example of where selective CO2 removal from a multicomponent gaseous stream is desirable is the processing of synthesis gas or syngas. Syngas is a mixture of hydrogen, carbon monoxide and CO2 that is readily produced from fossil fuels and finds use both as a fuel and as a chemical feedstock. In many applications involving syngas, the carbon monoxide is converted to hydrogen and additional CO2 via the water-gas shift process. It is then often desirable to separate the CO2 from the hydrogen to obtain a pure H2 stream for subsequent use, e.g. as a fuel or feedstock.
As man made CO2 is increasingly viewed as a pollutant, another area in which it is desirable to separate CO2 from a multicomponent gaseous stream is in the area of pollution control. Emissions from industrial facilities, such as manufacturing and power generation facilities, often include CO2. In such instances, it is often desirable to at least reduce the CO2 concentration of the emissions. The CO2 may be removed prior to combustion in some cases and post combustion in others.
A variety of processes have been developed for removing or isolating a particular gaseous component from a multicomponent gaseous stream. These processes include cryogenic fractionation, selective adsorption by solid adsorbents, gas absorption, and the like. In gas absorption processes, solute gases are separated from gaseous mixtures by transport into a liquid solvent. In such processes, the liquid solvent ideally offers specific or selective solubility for the solute gas or gases to be separated.
Gas absorption finds widespread use in the separation of CO2 from multicomponent gaseous streams. In CO2 gas absorption processes that currently find use, the following steps are employed: (1) absorption of CO2 from the gaseous stream by a host solvent, e.g. monoethanolamine; (2) removal of CO2 from the host solvent, e.g. by steam stripping; and (3) compression of the stripped CO2 for disposal, e.g. by sequestration through deposition in the deep ocean or ground aquifers.
Although these processes have proved successful for the selective removal of CO2 from a multicomponent gaseous stream, they are energy intensive. For example, using the above processes employing monoethanolamine as the selective absorbent solvent to remove CO2 from effluent flue gas generated by a power plant often requires 25 to 30% of the available energy generated by the plant. In most situations, this energy requirement, as well as the additional cost for removing the CO2 from the flue gas, is prohibitive.
Accordingly, there is continued interest in the development of less energy intensive processes for the selective removal of CO2 from multicomponent gaseous streams. Ideally, alternative CO2 removal processes should be simple, require inexpensive materials and low energy inputs, and be low in cost for separation and sequestration of the CO2. Of particular interest would be the development of a process which provided for efficient CO2 separation at low temperature (e.g. 0 to 10xc2x0 C.) from low CO2 partial pressure multicomponent gaseous streams.
Relevant Literature
Patents disclosing methods of selectively removing one or more components from a multicomponent gaseous stream include: U.S. Pat. Nos. 3,150,942; 3,359,744; 3,479,298; 3,838,553; 4,253,607; 4,861,351; 5,397,553; 5,434,330; 5,562,891; 5,600,044 and 5,700,311.
Other publications discussing CO2 clathrate formation include Japanese unexamined patent application 3-164419; Austvik and Loken, xe2x80x9cDeposition of CO2 on the Seabed in the Form of Clathrates, xe2x80x9d Energy Convers. Mgmt. (1992) 33: 659-666; Golumb et al., xe2x80x9cThe Fate of CO2 Sequestered in the Deep Ocean,xe2x80x9d Energy Convers. Mgmt. (1992) 33: 675-683; Morgan et al., xe2x80x9cHydrate Formation from Gaseous CO2 and water,xe2x80x9d Environmental Science and Technology (1999) 33: 1448-1452; Nishikawa et al., xe2x80x9cCO2 Clathrate Formation and its Properties in the Simulated Deep Ocean,xe2x80x9d Energy Convers. Mgmt. (1992) 33:651-657; Saji et al., xe2x80x9cFixation of Carbon Dioxide by Clathrate-Hyrdrate,xe2x80x9d Energy Convers. Mgmt. (1992) 33: 643-649; Spencer, xe2x80x9cA preliminary Assessment of Carbon Dioxide Mitigation Options,xe2x80x9d Annu. Rev. Energy Energy Environ. (1991) 16: 259-273; Spencer and North, xe2x80x9cOcean Systems for Managing the Global Carbon Cycle,xe2x80x9d Energy Convers. Mgmt. (1997) 38 Suppl.: 265-272; and Spencer and White, xe2x80x9cSequestration Processes for Treating Multicomponent Gas Streams,xe2x80x9d Proceedings of 23rd Coal and Fuel Systems Conference, Clearwater, Fla. (March 1998).
Methods are provided for the selective removal of CO2 from a multicomponent gaseous stream to provide a CO2 depleted gaseous stream having at least a reduction, e.g. 20%, in the concentration of CO2 relative to the untreated multicomponent gaseous stream. In practicing the subject methods, the multicomponent gaseous stream is contacted with an aqueous fluid, e.g. CO2 nucleated (or structured) water, under conditions of selective CO2 clathrate formation to produce a CO2 clathrate slurry and CO2 depleted gaseous stream. A feature of the subject invention is that a CO2 hydrate promoter is employed, where the CO2 hydrate promoter is included in the multicomponent gaseous stream and/or the aqueous fluid. The CO2 hydrate promoter serves to reduce the minimum CO2 partial pressure required for formation of CO2 containing hydrates (i.e. CO2 hydrates) as compared to a control using pure CO2 gas and water. The subject methods find use in a variety of applications where it is desired to selectively remove CO2 from a multicomponent gaseous stream.
Methods are provided for the selective removal of CO2 from a multicomponent gaseous stream to provide a CO2 depleted gaseous stream having at least a reduction, e.g. 30 to 90%, in the concentration of CO2 relative to the untreated multicomponent gaseous stream. In practicing the subject methods, the multicomponent gaseous stream is contacted with an aqueous fluid, e.g. CO2 nucleated (or structure) water, under conditions of selective CO2 clathrate formation to produce a CO2 clathrate slurry and CO2 depleted gaseous stream. A feature of the subject invention is that a CO2 hydrate promoter is employed, where the CO2 hydrate promoter is included in the multicomponent gaseous stream and/or the aqueous fluid. The CO2 hydrate promoter serves to reduce the minimum CO2 partial pressure required for formation of CO2 containing hydrates as compared to a control using pure CO2 and water. The subject methods find use in a variety of applications where it is desired to selectively remove CO2 from a multicomponent gaseous stream.